Estimating the degree of preferential flow to drainage in an agricultural clay till field for a 10-year period

Abstract. The conceptual understanding of the preferential water flow is crucial and hence understanding the degree of water percolating rapidly through vertical macropores, or slowly through the low-permeable matrix, is vital in order to assess the risk of contaminants like nitrate and pesticides being transported through a variably-saturated macroporous clay till to drainage. This study compared six different model concepts, using the dual-permeability module of the one-dimensional model DAISY, incorporating three different macropore settings and two different groundwater tables set as lower boundary conditions. The three macropore settings included vertical macropores supplying water directly to (a) drainage, (b) drainage and matrix and (c) drainage and matrix including fractures supplying water to the matrix in the saturated zone. The model study was based on ten years of coherent climate, drainage, and groundwater data from an agricultural clay till field. The estimated drainage obtained with the six model concepts was compared to the measured drainage. No significant discrepancies between the estimated and measured drainage were identified. The model concept with the macropore setting (b) exposed to groundwater fluctuations measured in the southern part of the field, gave the best description of the drainage. Bromide leaching tests were used to evaluate the mass balance of the model concepts. The estimated water balance of all six concepts revealed that 70 % of the precipitation input to drainage was transported via macropores. According to the results of bromide leaching simulation, 54 % of the drainage was estimated to be transported via vertical macropores being initiated in the plow layer.